1. Field of the Invention
The invention relates to highly sensitive physical disturbance or vibration sensing by determining the effects of polarized light as affected by compression of an optical gap material, such sensing mechanism being susceptible of considerable miniaturization.
2. Description of the Prior Art
There are a number of prior teachings that relate to the use of optical fibers and interferometric techniques in determining the presence and character of vibrations and other non-harmonic movements. U.S. Pat. No. 4,368,645 discloses one form of optical pressure sensor that utilizes a material known as a photoelastic medium for effecting light changes. The index of refraction along x and y axes of the photoelastic medium will be different for forces applied to the axes and a relationship can be set up to determine the amount of difference in accordance with an applied pressure. The device utilizes circularly polarized light passing through a photoelastic medium, which light is then separated into two components for detection. In final analysis, two polarization components of light emerging from the photoelastic medium are individually measured and then processed to output a final determination that is related to pressure measurement. A related U.S. Pat. No. 4,321,831 in the name of Tomlinson et al. extends the teachings. That is, the device functions with linear polarized light and a birefringent plate to determine generally the applied pressure.
Another U.S. Pat. No. 4,556,791 teaches the use of an elasto-optic member affecting light as a function of an external stress to the elasto-optic member. This device divides light emanating from the elasto-optic material into orthogonal components aligned in bisection of the stress axis. The orthogonal components are photodetected with outputs applied to a difference amplifier, and the voltage difference is then related to the elasto-optic sensor. Finally, U.S. Pat. No. 4,518,857 discloses a device using an elasto-optic sensor that will provide output indication as to pressure while also compensating for temperature and pressure of operation. This device utilizes a pressure sensitive birefringent element which will cause fluctuations in light phase difference in response to applied pressure. Orthogonal components of polarized light are then processed to derive an output indication related to the differential phase shift established by the birefringent element.
U.S. Pat. No. 4,599,711 provides teaching of a fiberreflector gap in contact with a reflector 17 that is responsive to pressure variations, with the opposite side of the fiber-reflector gap connected to fiber optic cables, one transmitting and two of different size receiving light. Movement of the reflector element will then vary the area illumination of the different diameter receive fibers thereby to modulate the intensity of the received light. Finally, U.S. Pat. No. 4,642,458 teaches a polarimetric fiber sensor that consists of a birefringent optical fiber having the end silvered to provide reflection. Source light is continually reflected from the distal end reflector whereupon changes in the modal birefringence are monitored to detect changes in the physical parameter affecting the distal end, i.e., the reflecting end.